Light-sensitive compositions and plates for photolithography



l atent ed Nov. 6, 195i LIGHT-SENSITIVE coMr'osITIoNs AND PLATES FORPHOTOLITHOGRAPHY Frederick H. Frost, Portland, and Frederic E.-

Brinnick, Westbrook, Maine, assignors to S. D. Warren Company, Boston,Mass;, a corporation of Massachusetts No Drawing. Application April 26,1943; Serial No. 23,403

Claims.

This invention relates to the art of photolithography and particularlyto sensitized solutions or compositions for preparing light-sensitiveplates and to the plates so prepared.

In the usual practice of photolithography a suitable base, such as agrained metal plate or a suitable coated paper of the kind well-known inthe art, is coated with a thin even layer or film of a sensitizedsolution such as an aqueous solution of egg albumin sensitized withammonium dichromate, and is then dried. The plate so treated is exposedthrough a mask or negative to the action of actinic radiation, as froman arclight, which effectively tans or insolubilizes the film in theareas exposed to the light but leaves unchanged or still soluble thefilm on the unexposed areas. The exposed plate is next wiped over withgreasy developing-ink which coats the entire surface; and then it iswashed in water with rubbing which loosens the water-soluble areas ofthe film and removes them together with the ink overlying those areas,but leaves on the plate the exposed water-insoluble areas of the filmtogether with the ink overlying them. After the preceding treatment theplate is ready to be used for planographic printing. The unexposed areasare kept wet with aqueous liquid which repels ink when the surface isrolled with an inking-roller coated with lithographic printing-ink. Atthe same time the inked areas repel water and are not wet by the aqueousliquid but are readily wet by the lithographic ink on the roller and areinked thereby. When the inked plate comes into contact with anothersurface it transfers thereto the ink in a pattern reverse to that on theplate. The surface so printed upon may be a paper sheet, but in mostcases is an offset blanket which in turn transfers the print to a papersheet in a pattern like that on the plate to complete the printingoperation. The plate is then repeatedly dampened, inked, and

printed from until the desired number of copies have been made.

In the past the sensitized coating material generally used has been onewhich dries as a solid film, e. g. albumin. After exposure of such afilm through a negative all unexposed portions must be washed away. Anyunexposed film inadvertently left on the surface causes scumming andconsequent toning or discoloration in the unimaged portions of theprints made from the plate. Even when metal plates are used it isdifiicult to be certain that all unexposed film has been removed;paper-base planographic plates from unexposed film.

In the present invention, on the other hand, the sensitizedimage-forming material in contradistinction to the solid film-formerspreviously used is one which before being exposed to action of light isnot a solid film-former but instead is a liquid or semi-liquidsubstance. The portions of such sensitized substance which are exposedto actinic radiation are oxidized to a coherent solid film, but theunexposed portions remain liquid or semi-liquid and are removable withcomparative ease. Moreover, the exposed areas of the film have acomposition sufficiently similar to that of printing-ink that the imagewill pick up lithographic ink quickly without prior treatment withdeveloping-ink.

In the past, use of developing-ink has been general in order (1) toensure water-repellency of the image, and (2) to prevent damage to orobliteration of the image during the scrubbing operation to clean theunexposed areas. The application of developing-ink, however, and itssubsequent removal along with unhardened film from unexposed areastogether comprise a somewhat disagreeable procedure. Elimination of thene- 7 cessity for use of developing-ink is much to be desired. This thepresent invention achieves.

By the present invention a sensitizing composition is provided in whichthe usual albumin or other solid film-forming material is replaced by aliquid or semi-liquid fatty or oily material which is capable ofreducing bichromate under the influence of light. The preferred fatty oroily materials are highly unsaturated substances,

ve. g. tung oil, but material of a lower degree of unsaturation isusable. For the purpose of this disclosure any fatty or oily material isconsidered to be an unsaturated substance if it will reduce bichromateunder the influence of light. They are generally naturally occurringglycerides of higher fatty acids, modifications and derivatives thereofand their synthetic equivalents. The sensitizing compositions of theinvention likewise contain a light-sensitive chemical such as a solublehexavalent chromium compound, i. e. chromate or bichromate and also asoluble ion'izable p'hosphate'such as an ammonium phosphate.

The presence of ammonium phosphate or equivalent phosphate capable ofsupplying phosphate ions seems to be essential. In the absence usuallyare still more diflicult to free completely of phosphate the backgrounddoes not clean up readily and a sharp image is not obtained quicklyunless developing-ink is used. When phosphate is included in thesensitizing solution, however, the unexposed areas are easily cleaned,and

the image will take ink readily without any treatment withdeveloping-ink. Usable phos- 3 phates include water-soluble inorganicphosphates, organo-metallic phosphates, and organic phosphates such asthe amine phosphates which can exist in the coating in water-soluble,readily ionizable form. The pH value of the sensitized coating is ofcourse influenced by both the kind and quantity of phosphate used and bythe kind and quantity of hexavalent chromium sensitizer used. Thosephosphates are preferred which in cooperation with the chromiumsensitizer used yield a solution having a pH value between 3.0 and 7.0inclusive. Solutions more a'cid'thanindicated by a pH value of 3.0 areundesirable'because of their marked efiect on the base plate. On theother hand, solutions more alkaline than indicated by a pH value of 9.0are unusable because of their deleterious eflect upon the image. Mildlyalkaline coatings having pH values b'etween 7.0 and 9.0 are'generallyusable but are less'desir'able than th'ose of not over 7.0 pH value. Itis not,"however,essential to use'a'n acidic phosphate in the' solution,for the chromium se'n'si'tizer chosen 'for' 'use may be one likeammonium dichromate which exerts'a buffering action "to keep the'pHvalue of the solution at 7.0 or below "-In such a caseaphosphate asalkaline as trisodium phosphate can be used with good'r'esults,thouglrnaturally it is possible to use so mucli'of-ahighly alkalinephosphate that 'theresulting solutionmay become too alkalinefor-satisfactory use.

The sensitizing chemical preferred for use is a 'h'exavalent chromiumcompound such as ammonium chromate or dichromate. Other soluble'chromates and dichromates are usable but are" soiiiewh'at slower intheir action than are the ammonium" salts. Y

' The oilyor "greasycomponent of the sensitized coating may beany-drying or semi-drying oil or aderivative thereof particularly awater-soluble 40 derivative *such as' a sulfonated oil. -Naturallyoccurring oils may be used such as raw tung' oil, or the oilmay bemodified as bylieating such as heat-bodiedtung-"oil. On" the other handthe oil may'be-" one-whieh hasbeen treated to increase itsdegree of-unsaturation, e; g. dehydrated castor oil. Likewise are usable variouscompounds of the'unsa"turated fatty acids such as 'rsoi bitan oi'ea'teor 'polyoikyethylene d'eriv'atives' of that or "similar compounds.

' The oily or greasy materials are, of course, fapplied infan' aqueous"medium' along with the fph'o'sphate' and dichromate. If the materialsar e -";not intrinsically soluble or dispe'rsible in water they may beemulsified inwater by means of known emulsifying-agents. H

"(The Y chief objectsof the invention may be ac- Y 'complishedby use ofa sensitizingsolution com- ""pr'i'sing only clichromate, phosphate, andunsatu- "rated oil disIJersed" in water. In s'omefcases, hcweven itmaybe desirable to hardemupthe "sensitizedcoating somewhat, for'instance ifthe plate is' tobe stored orha'ridled'before use. If

suchi thef'desire" a smallproportion of easily soluble film-'forrningsubstance may be added to the-sensitizing solution, e. g. dextrin,mesquite gum, or the like. It is found that such easily soluble gum doesnothinder the removal of unexposed coating from the plate but on thecontrary actually seems to facilitate such removal. Atthe same time,however, addition of such gum in small proportion'to the" sensitizingsolution does-notmaterially'afiect the life of the image in the exposedportions of the plate. As'the prportionof suchguni is increased theimage-life 30 'soluble 'pho'sphate of the metal.

50 e d 9 fatty begins to decrease, but quantities of gum up to abouttwice the weight of oily material may be used without hurting theimage-life seriously.

Permissible variation in the ratios of dichro- 5 mate to Oily materialin'the sensitizer appears to be considerably greater than can betolerated in .i the ratio of dichromate to albumin in conventionalalbumin sensitizing solutions. While 'limitingratios'have beendetermined we have "obtained goodresults with ratios of dichromatecalculated as ammonium dichromate to oily material 'varying 'aswidely asfrom 0.25:1 to 2:1

by weight.

The preferred concentration of phosphate,

reckoned as mono-ammonium orthophosphate,

ranges from 0.5% to 3.0% by weight of the sensitizing solution, with thelower concentrations being'used on metal plates and the higherconcentrations on paper-base plates.

0 The base plates which are -suitable for use with the sensitizingsolutions include the metal plates, e's'gLizinc or'alumin'um, commonlyused for lithographic printing and the various commercially availablepaper 'base plates used for planographic .g'printingdncluding parchmentand coated papers.

-Especiallysuitable'are the'coated paper plates which contain a solublemultivalent metal compound capable of reacting with the'solublephosphatein-thesensitizer solution to 'form a less Such plates are.disclosed' in the applications of Stephen V. -Worthen' Serial Nos.747,138 and 758,215, filed May'9, 1947 and June30, l947,"both' nowaband0ll'8d."'II1 the case of a plate containing alead salt such asleadacetate it is probable, sinceiead chromate is less soluble thanlead-phosphate; that lead chromate is formed by interaction of" the"lead salt with the'chromate of thesensitizer solutionand the resultsare less desirable than when the phosphate'of the metal, e. g. zinc; isless soluble than the chromate thereof.

Typical oil emulsions suitable for use in practicing the'invention aredescribed below.

Example 1 ff'run oil arts was heated with 10 parts ot cetylalcohol (anagent to stabilize the emulsion) until the latter dissolved. Thissolution was then added to 2l5 parts of water containing '10 parts of awater-soluble polyoxyethylene derivative of lina'cid (an emulsifyingagent). The

; mixture was pass'ed ;through a colloid mill and formedastable.emulsion which was readily dilutable with water.

5 "Example 2 (if water eontaining'iparts' o'f t e mmxyerfiyapeadderivative ofsorbitari mo miaurateand Smarts of "sodiumrri'iired'fatty alcohol' 's'iilfate'femiilsiiying agents) The mixturewas then 'pas'sedthrou'gh afcolloid remand formed a stable'emulsioiiwhich "was "readily dilutable with water.

' Example 3 Linseed onjrqo' pairs ana's rntafi-rirafiaststkte oeartswere mixedtogether and mixedwith' 215 parts of water containing 5 parts:of a water- I hylene derivative of sorbitan T' rrionostearate and 5-parts of sodium inixed fa'tty alcohol Sulfate. The mixture'was PASS edthrough a colloidmill and iormed a stable emulsionwhieh was readny'dilutable 'with'water.

For the particular oils used'm the'i'r'eeaing 5. specific examples otherunsaturated oils or semiliquid organic materials capable of reducinghexavalent chromium and of thereby forming a solid film may besubstituted. Soy-bean 1 oil, rapeseed oil, perilla oil, oiticica oil andthe like are all "satisfactory for use. Any of these oils may be usedwith the emulsifying agents of any of these previous examples. Theemulsions of these particul'ai'examples are all very stable and containstabilizers.- However, far less stable emulsions can be satisfactorilyused. If the emulsion is to be used immediately after preparation it ismerely necessary that the oil shall remain emulsified for a matter ofminutes. Naturally, it is more con- 'venient to have the emulsion stableenough so that stock emulsions can be prepared and stored.

The particular emulsifying agent usedin emulsifying oils to be used inpracticing the invention in itself forms no part of the invention; Thereis some advantage in using an emulsifying agent, like that shown inExample 1, which itself can be oxidized and rendered insoluble by theaction of chromate and light. But emulsifying agents which are notaffected by the conditions of plate exposure likewise are usable withcompletesatisfaction. When very efficient wetting agents are used toemulsify the oil large excesses thereof over the quantity needed toaccomplish the emulsification should be avoided. If too large an excessis used undesirable penetration of the sensitizing solution intopaper-base plates may result; furthermore, too high a. proportion of, a

--very efficient wetting agent may tend to shorten the life of theprinting image formed. On the other hand an oil film deposited from anemulsion and .containing an emulsifying agent undoubtedly isconsiderably easier to remove from the unexposed areas of a plate thanwould be an oil film merely spread on the surface or deposited fromsolution in a volatile solvent.

As has been before pointed out it is not necessary to use emulsified oilunder the invention. Derivatives of oils which are soluble in water suchas the polyethylene derivatives and sulfonated derivatives may be usedwith great satisfaction.

On the other hand, it is possible to use. emulsifying oil varnishes aswell as simple oils; that is an oil in which a resin such as copal, anoilsoluble phenolic resin, 'or an oil-soluble alkyd resin has beendissolved or cooked may be emulsifiedand used under the invention inplace of a simple oil emulsion.

7 Some specific sensitizing solutions are shown in the followingexamples.

Example 4 A sensitizing solution containing 10 parts of ammoniatedsulfonated oil, 10 parts of monoammonium phosphate, and 7.5 partsammonium dichromate in 470 parts of water was applied to a coated paperlithographic plate having a base coat of insolubilized casein and clayand having been wash-coated with a solution of guar gum and zinc acetateas described in the applications of Stephen V. Worthen referred toabove. The so-sensitized plate was exposed under a negative to lightfrom an arc-lamp. The exposed plate was wiped with a cotton swabdampened with water which cleaned off the unexposed areas. The plate wasthen put on an offset duplicator where it gave satisfactory prints.

Example 5 Tung oil was heat-treated until its viscosity was increased tovalue M on the earaner'fiioldt scale of oil viscosity standards. Thisbodied oil was emulsified like that in Example 2 preceding and thendiluted with water until the oil concentration was 10%. Then 100 partsof the emulsion .was mixed .with 380 parts of water having dissolvedtherein 10 parts ofmono-ammonium phosphate and 7.5 parts of ammoniumdichromate. This solution was applied to the same. kind of coated paperplate usedv in Example 4. The sensitized plate was then. further treatedexactlyas in said Example 4. The image took ink quickly without anydeveloping treatment and the .unexposed areas remained satisfactorilyclean.

Example 6 The emulsion of Example 1 was diluted with water until theoil'concentration was 10%. A sensitizing solution was prepared .bymixing .100 parts of this 10% emulsion in 570 parts of water in whichwere dissolved.17.5 parts of mono-ammonium phosphate, 15 parts ofammonium dichromate, and 8 parts of mesquite gum. This solution was usedtosensitize a coated. paper plate like that of Example 4 andthe platewas further treated as in Example 4. The results were satisfactory inevery Way.

Example 7 100 parts of the emulsion of Example 7 (containing 10 parts oftung oil) 10 parts of diammonium phosphate 7 .5 parts of'ammoniumdichromate 400 parts of water Example 9 Same as Example 8 except for thesubstitution of 10 parts of mono-potassium phosphate in place ofdi-ammonium phosphate.

Example 10 Example 11 100 parts of the emulsion of Example 7 5 parts ofmesquite gum 10 parts of ethyl ammonium phosphate 8 parts of ammoniumdichromate 400 parts of water Example 12 100 parts of the emulsion ofExample 7 3 parts of mesquite gum 10 parts monomethylamine acidphosphate 8 parts ammonium chromate 7 400 parts of water applied 110'?the samestype :ofpguar ;gum- ,acetate'treatedcoated. plate used;-.=in:Example .4.

aofl- Examplescaefl was and zinc :ef': the .solutions Thesensitizedplatesiwere then dried; exposed to a..- negative, and thenIrr-i-place .of; the. phosphatesimentioned in.:,the

I preceding. examples-1 the following phosphates havealso beenused-satisfactorily::sodiumzmetaphosphate, lxsodium: pyrophosphate, ,2tri-sodium phosphatatammonium hexametaphosphate, di-

potassium phosphate',1ipentaethyl potassium tripolyphosphate,mono-isopropyl sodium phosphate, dimethyl amineracid phosphate,monoethanolamine acid phosphate.

Example '1 3 .100. parts of. the emulsion of Example 7 =parts ofrnono-ammonium phosphate 3.5. parts ofammonium dichromateipartsofmesquite gum 400 partsof water .Thissolution was. appliedbymeansof a whirler to a (grained aluminum plate. and .to. a :freshly"cleaned"grainedflzincmplate. .The. plates were dried, exposed throughanegativeto light, wiped with a damp sponge,.and.then placed in anoffset duplicator where they ran satisfactorily in "respect todefinition, -'-backgroundclarity, and

image-life.

The oily 'or fatty componentof our sensitizer compositions are believedto-be all embraced by thedescription or.definition--that they are nor--mally structureless unsaturatedfatty acid compounds which arecapablein-therpresenoe of hexavalent chromium and actinic. light. offorming coherent films.

We claim:

v 1. .A sensitizer. composition for photosensitive planographic printingplates comprising .aan aqueous vehicle, aaewater-soluble hexavalentchromium compound vof the group .consistingrof chromates anddichromates, a'liquid.v to' similiquid, unsaturated, organic, oily tofatty material capable ofreacting with bichromate in the presence ofactinic light to form a coherent solid film and a water-solublephosphate, the weight ratio of said chromium compound to said materialbeing within the range. from 1 to 8 parts of the chromium compound to 4parts of said material, and said composition containing from .5 to 3percent by weight of said phosphate and having a pH value withinthe-range from 3 to 9.

2. A sensitizer composition .as defined-in claim least the surfacethereof coating'onsaid surface consisting-of the lfinrwhich :the. saidmateriallis an oil otgth group consisting-of. ,drying. andvsemi-dryim0115:

75.3. A sensitizes-composition as .defined-inrclaim 2 in which-:the saidmaterial is tung oil.

.4.- Asensitizer composition as defined inlclaim 2 in. whichthecomposition comprises-an -emulsiifyingjagent for saidgnaterial.

.25. ;A-.sensitizer-composition as defined inclaim 1 :in; which the:said; material is ,an:intrinslcai1y wateridispersible: derivativeof .anunsaturated ifattyacid.

6. A-zsensitizerlcomposition-as defined in claim 5 which:ltheziemulsiiying agentris; a' water- -solublezderivative of .anunsaturated f atty, acid.

7.x Asensitizer. composition as defined inclaim 1 comprising awater-soluble" organic; hydro --philic;:film-:forming,inon proteinhydrocolloid in 1 quantity not exceeding twice the weight :of saidmaterial.

" 8:. A rphotosensitive plate. comprising. a .base

:and. a-.,coating thereon consisting of the dried l1'BS1d118 of ithe'sensitizer. composition :deflneds: in

claim 1.

1.79.; A photosensitive "plate comprising a base containingazwater-soluble, water-stable salt of a. metal having, a :.valencegreater than 1, in 'at and. ac photosensitive dried --residueiof'thecomposition definedin claim 1.

*210aA planographic,printing-plate comprising a base anda partialcoating thereon intheform of a .configuration,.-said coating. being .thedried andinsoluble residue of the compositionadefined in claim 1.

"FREDERICK. H. FROST.

.FREDERIC"E. BRINNICK.

- REFERENCES CITED UNITED STATES PATENTS Number Name 1 Date 1 55,592vWillis June 12,1866 1,574,356 vBeebe Feb. 23,1926 1,574,357 Beebe et alFeb..23',i1-926 1,587,274 Beebe et a1 June 1,1926 1 1,618,931 KammererFeb. 22,1192! FOREIGN PATENTS Number Country I Date 205,092 GreatBritain .Dec. 11,1924 408,097 Germany .Jan.,2, 1925 228,377 GreatBritain .Feb. 5,4925 598,799 Germany, June-18,1934 517,686 Great BritainFeb. 6, 1940

1. A SENSITIZER COMPOSITION FOR PHOTOSENSITIVE PLANORGRAPHIC PRINTINGPLATES COMPRISING AN AQUEOUS VEHICLE, A WATER-SOLUBLE HEXAVALENTCHROMIUM COMPOUND OF THE GROUP CONSISTING OF CHROMATES AND DICHROMATES,A LIQUID TO SIMILIQUID, UNSATURATED, ORGANIC, OILY TO FATTY MATERIALCAPABLE OF REACTING WITH BICHROMATE IN THE PRESENCE OF ACTINIC LIGHT TOFORM A COHERENT SOLID FILM AND A WATER-SOLUBLE PHOSPHATE, THE WEIGHTRATIO OF SAID CHROMIUM COMPOUND TO SAID MATERIAL BEING WITHIN THE RANGEFROM 1 TO 8 PARTS OF THE CHROMIUM COMPOUND TO 4 PARTS OF SAID MATERIAL,AND SAID COMPOSITION CONTAINING FROM .5 TO 3 PERCENT BY WEIGHT OF SAIDPHOSPHATE AND HAVING A PH VALUE WITHIN THE RANGE FROM 3 TO 9.